The invention pertains to the field of batteries. Specifically, the invention deals with novel aqueous rechargeable batteries that employ insertion compounds for the electrode materials.
The demand for rechargeable batteries with higher gravimetric and volumetric energy densities has been increasing in recent years. Potential applications for these energy sources range from consumer electronics devices to motive power for electric vehicles. To meet these demands, a variety of novel or improved electrochemical systems are being developed. Several such systems have recently been introduced on a commercial basis, and include products based on nickel-metal hydride (Ni-MH) or on lithium ion (also known as xe2x80x9crocking chairxe2x80x9d) systems. The former is an example of an aqueous rechargeable battery product, while the latter is an example of a non-aqueous product.
Aqueous rechargeable battery systems have been used commercially for decades. Common uses include smaller Ni-Cd products for household electronics devices and larger Pb acid products for SLI (starting-lighting-ignition) requirements for automobiles. Typically, aqueous battery products share several advantages over non-aqueous battery products. Being water based, the contents of the battery generally cannot ignite and burn. Thus, in abuse situations, aqueous batteries offer a relatively low risk of fire. (Of course, the risk of fire and/or explosion as a result of hydrogen generation in certain aqueous systems is well known). Additionally, aqueous electrolytes have ionic conductivities that are typically from 2 to 3 orders of magnitude greater than those of non-aqueous electrolytes at a given temperature. Thus, it is possible to design high rate (power) aqueous batteries with much thicker electrodes than those required for a corresponding high rate non-aqueous battery. The ability to fabricate high rate batteries, using relatively thick electrodes, translates into easier fabrication and easier prevention of defects with a corresponding reduction in cost. Finally, aqueous electrolytes are generally preferred over non-aqueous electrolytes from an environmental viewpoint.
Non-aqueous systems, on the other hand, have the advantage that they are not limited by the electrochemical stability of a water based electrolyte. Thus, such systems may operate at relatively high cell voltages ( greater than 3 volts), resulting in batteries with high energy densities. For example, battery systems employing lithium metal anodes often have theoretical energy densities of order of several hundreds of Wh/kg or Wh/l. In practice however, the safety characteristics of lithium batteries have limited both the practical energy densities obtained as well as the maximum battery size in commercial products to date.
Recently, lithium batteries based on lithium ion or xe2x80x9crocking chairxe2x80x9d electrochemistries have entered the marketplace. These electrochemistries employ two suitable lithium insertion compounds as the active electrode materials and a non-aqueous electrolyte. Typically a carbonaceous material (partially graphitized) is employed as the anode, and a lithium transition metal oxide is employed as the cathode. During a discharge of the battery, lithium is removed from the host anode insertion compound and is inserted into the host cathode insertion compound. On recharge, the reverse process occurs. No plating process of a transported ionic species is fundamentally involved. The battery voltage is determined by the difference in the chemical potential of lithium in the two host electrodes, which is on average about 3xc2xd electron volts. Lithium ion batteries thus offer high voltage with corresponding high energy densities, and these systems can cycle extremely well (over 1000 cycles).
Commercial lithium ion batteries can deliver energies of order of 200 Wh/l and 100 Wh/kg. This is achieved, in part, by using a minimal amount of non-aqueous electrolyte in the battery. Unlike some electrochemistries (eg. Pb acid, Nixe2x80x94Cd), the Li ion electrolyte does not participate in the reactions on charge or discharge and merely serves as a conduit for Li ions between the electrodes. The low ionic conductivity of the Li ion electrolyte however requires that thin electrodes (of order of 100 xcexcm thick) be used in the battery construction. With the use of very thin electrode substrates (eg. Al or Cu foil) and thin separators however, a relatively high loading of active electrode material can still be obtained in the fabricated battery. Approximately 45% of the overall weight of small cylindrical cells (eg. 4/3 A size) can be active electrode material.
The safety of lithium ion batteries is significantly better than that of similarly sized lithium metal batteries. There is still however a risk of fire or explosion under some types of mechanical or thermal abuse. This poses a problem for the commercialization of larger batteries or battery arrays. Also, those skilled in the art are aware that the risk of fire during abuse situations places limits on the deliverable capacity of such batteries. For instance, the amount of lithium that can be removed reversibly from commercial LiCoO2 based cathodes is significantly greater than that actually used in practice for reasons of safety.
There are other disadvantages associated with conventional non-aqueous lithium ion type electrochemistries. The thin electrode requirement means that costly separator and foil current collectors must be used. The thin electrode assembly is correspondingly more complex. The active materials used in the electrodes must obviously be significantly smaller than the electrode itself. Thus, fine electrode powders (with a corresponding higher reactive surface area) may need to be used even though large particles may be sufficient for a given discharge rate.
Although the fine electrode powders are generally stable in air, a significant amount of water can be adsorbed onto the large surface area presented by such powders. Additionally, the electrolytes used in lithium ion electrochemistries are also generally hygroscopic. Since it is detrimental to include water in an assembled battery, many fabrication steps involve water removal or shielding from moisture in the air (typically in dry room environments).
Another less well known problem arises from the instability of many common materials to oxidation at the high operating potentials at the typical lithium ion cathode. Fortunately, aluminum is an inexpensive material that is acceptable for use as hardware at the cathode potential for most but not all lithium ion electrochemistries. However, a significant problem can arise due to the presence of certain impurities in the cathode materials themselves. The presence of even one small particle of an oxidizable metal contaminant (such as copper, stainless steel, iron) in the cathode can result in the development of an internal short in the battery. At the high operating voltages of such batteries, these contaminants can dissolve and plate over to the anode, creating an electrically conducting contaminant bridge between the electrodes. The thin separators (approx. 25 xcexcm) employed in such batteries are not completely effective in preventing such internal shorts. Even with stringent quality control and cleanliness procedures, it is not uncommon in the applicant""s experience to obtain from 5 to 10% internal shorts in developmental 4/3 A batteries. Those skilled in the art who are aware of this problem will appreciate the difficulties that this will pose in fabricating large defect free batteries.
The choice of appropriate insertion compounds is fundamental to the construction of a lithium ion battery. Currently, lithiated transition metal oxides including LiCoO2, LiNiO2, LiMn2O4 (described in U.S. Pat. Nos. 4,302,518 and 4,312,930) and the like are among those favoured as cathode materials, while partially graphitized carbon or graphite (described in U.S. Pat. Nos. 4,702,977 or 5,028,500 and Japanese Patent Publication No. 57-208079) are favoured as anode materials. Such materials are popular for reasons of voltage and their reversible capacity. Over recent years however, a range of Li insertion compounds has been considered for use as either electrode in non-aqueous batteries including transition metal chalcogenides (eg. MoS2, TiS2), chevrel compounds (Mo6S8), other transition metal oxides (V2O5, MnO2), WO2, and so forth. Many of these compounds can have significant amounts of lithium inserted and extracted reversibly. LiNiO2 and LiCoO2 both can cycle over 140 mAh/g of lithium reversibly (Ohzuku et al, Chemistry Express, 7, 193 (1992)). A form of LiMnO2 synthesized at low temperature can reversibly cycle up to 190 mAh/g of lithium. (J. R. Dahn, U. von Sacken, and C. A. Michael, Solid State Ionics 44, 87 (1990) and J. N. Reimers and J. R. Dahn, J. Electrochem. Soc. 139, 2091 (1992) respectively) Li-graphite or LiC6 can cycle up to 360 mAh/g of lithium reversibly under some conditions. (as in U.S. Pat. No. 5,130,211 for example)
Non-aqueous xe2x80x9crocking chairxe2x80x9d type batteries employing an insertion species other than lithium have been considered by those skilled in the art. Insertion compounds are known to exist for many inserted species belonging to the group of alkali metals and alkaline earth metals in general. (The group of alkali metals includes the group Ia elements Li, Na, K, Rb, Cs and Fr while the group of alkaline earth metals includes the Group IIa elements Ca, Sr, Ba and Ra). However, competitive non-aqueous xe2x80x9crocking chairxe2x80x9d type rechargeable battery systems have not been developed to date using inserted species other than lithium. (In addition to being larger atoms than lithium and hence more difficult to diffuse, certain other members of the group of alkali metals are even more of a fire hazard than is lithium. This would not be a direct issue in an aqueous battery.)
Although not traditionally viewed in this sense, the nickel electrode of a Ni-Cd battery is essentially a hydrogen insertion compound. With similar thinking, a Ni-MH battery is essentially a xe2x80x9crocking chairxe2x80x9d battery using two hydrogen insertion compound electrodes. In this sense, practical competitive aqueous xe2x80x9crocking chairxe2x80x9d type rechargeable batteries exist wherein the inserted species is hydrogen.
Alkali metal and alkaline earth metal insertion compounds have been considered for use in other aqueous applications, such as ion exchange media or sensor electrodes (such as in Japanese Patent Application Laid Open No. 52023692 or Y. Miyai et al., Nippon Kaisui Gakkaishi, 41(231) 152-6 (1987) respectively). However, aqueous xe2x80x9crocking chairxe2x80x9d batteries using these compounds have not been considered in the art. This might be presumed to be a result of the known instability of many common lithium insertion compounds in water.
The inventors have invented useful aqueous xe2x80x9crocking chairxe2x80x9d type rechargeable batteries using insertion compound electrodes and inserted species that are conventionally considered only for use in non-aqueous batteries. Based on the meaning intended by the term xe2x80x9crocking chairxe2x80x9d, the batteries of the invention employ a cathode comprising a first insertion compound with inserted species A, and an anode comprising a second insertion compound also with inserted species A. On recharge of the battery, species A is extracted from the first insertion compound while concurrently species A is inserted into the second insertion compound. (The battery of the invention employs an electrolyte comprising a salt of A dissolved in an aqueous solvent mixture, thus providing an ionic pathway between anode and cathode for species A. For the batteries of the invention, A is a member of the group of the alkali metals and alkaline earth metals.
A basic electrolyte can be employed wherein the pH is greater than 7. This may allow insertion compounds to be used in the battery of the invention that would not normally be stable in pure water.
The inserted species A for the battery of the invention may be lithium, and either or both of the first and second insertion compounds may be a lithium transition metal oxide. The first and second insertion compounds may even be of the same host structure, but have differing amounts of inserted species A.
Lithium manganese oxides or lithium vanadium oxides may be employed as either the first or second insertion compound. The spinel LixMn2O4, wherein x is a number which can range from 0 to about 2, can be used as either the.active cathode or the anode material or both. Lixcex3MnO2 with a xcex3-MnO2 structure, wherein y is a number which can range from 0 to about 1, can be used as an anode material. LizVO2(B), wherein z is a number which can range from zero to about 0.5, can also be used as an anode material.
The salts employed in the electrolyte of a battery using lithium as the inserted species may be LiOH, LiCl, LiNO3, Li2SO4, or Li(acetate).
An electrolyte comprising more than one salt may be employed. In order to simultaneously obtain a basic electrolyte having a certain concentration of ionized species A but, additionally, a lower concentration of hydroxide ions, a hydroxide salt may be used in addition to a salt of A. In a battery using lithium as the inserted species, the salt can be LiCl, LiNO3, Li2SO4, or Li(acetate) and the additional hydroxide salt can be LiOH wherein the concentration of the former is greater than that of the latter.
Other aspects of the invention include a method for making a rechargeable battery comprising: selecting a first insertion compound capable of insertion with species A wherein A is a member of the group consisting of the alkali metals and alkaline earth metals; selecting a second insertion compound capable of insertion with species A; selecting an electrolyte comprising a salt of A dissolved in an aqueous solvent mixture in a concentration such that both first and second insertion compounds are kinetically stable therein over some respective ranges of inserted A; and constructing a battery comprising a cathode, the cathode comprising the first insertion compound, an anode, the anode comprising the second insertion compound, a total amount of inserted species A, separation means for electrically separating the cathode from the anode, cathode current collector means in electrical contact with the cathode, anode current collector means in electrical contact with the anode, and a container.
A portion of the inserted species A in the range from zero to the total required amount can be inserted into the first insertion compound prior to constructing the battery. Additionally, a portion of the inserted species A in the range from zero to the total required amount can be inserted into the second insertion compound prior to constructing the battery. Alternately, a portion of the inserted species A in the range from zero to the total required amount can be inserted into either the first or the second insertion compounds by electrochemical means, the portion originating from the salt of A.
An electrolyte can be selected that further comprises an additional hydroxide salt, the concentration of said additional hydroxide salt being less than the concentration of the salt of A.
Finally, the first and the second selected insertion compounds selected can be the same.